1. Field of the Invention
This invention relates in general to data storage disk drives commonly referred to as disk drives, and in particular to a system and method for reducing track seek time in such disk drives.
2. Description of the Background Art
In data storage systems such as magnetic disk drives, digital information is magnetically stored upon a surface of a magnetic medium such as a magnetic storage disk in a set of concentric circular patterns called "tracks". The digital information is represented by selectively polarizing the surface of the disk. When this information is read back from the disk, the magnetic polarization of the medium is sensed and converted to an electrical output signal. The read and write operations are performed by read/write electronics in conjunction with a read/write head which "flies" over the surface of the rotating disk and provides an output signal.
Typically, storage disks of a disk drive are stacked in a "disk stack" (also known as "disk pack") which are mounted for rotation together on a single spindle. Each side of each disk in the disk stack has a surface which is usually used to store information. Each surface of a disk in the disk stack is usually exposed to at least one head responsible for reading and writing information on that particular surface. Typically, all the magnetic heads which are mounted on an actuator arm move in tandem radially over the surfaces of the disk so that they are all at the same approximate disk radius at the same time.
In order to accurately move a magnetic head to a desired track and position the head over that track a servo system is utilized. The servo system performs two distinct functions known as a "seek" or "access" function and "track following" function. During the "seek" operation the servo system moves a read/write head to a selected track from a previous track or from a park position as quickly as possible. When the head reaches the desired track, the servo system begins a "track following" operation in which it accurately positions the head over the centerline of the selected track and maintains the head in that position as successive portions of the track pass by the head.
It is important to note that during a seek operation the actuator arm where the head is mounted, in general, is moved as fast as possible so as to minimize the time required for that operation. Since the seek time is one of the most important factors considered in measuring the overall performance of a disk drive, it is essential to minimize the time it takes for carrying out the seek operation as much as possible.
In order to read and write data from the correct location in the disk pack, the data sectors in the disk pack are identified by a cylinder address, head address and sector address (CHS). A "cylinder" identifies a set of specific tracks on the disk surfaces in the disk pack which lie at equal radii and are, in general, simultaneously accessible by the collection of heads. The head address identifies which head can read the data and therefore identifies the disk that the data is recorded on. Each track within a cylinder is further divided into "sectors" for storing data and servo information.
Many modern disk drives also use a concept known as zone bit recording (ZBR) as taught by Hetzler in U.S. Pat. No. 5,210,660 in which the disk surface is divided into radial zones and the data is recorded at a different data rate in each zone. The addition of zones requires expansion of the cylinder, head, sector (CHS) identification scheme (addresses) to a zone, cylinder, head, sector (ZCHS) identification scheme.
Some disk files have servo information only on a dedicated surface on one disk in a disk stack. However, many modern disk drives use a servo architecture known as "sectored servo" (also referred to as "sector servo") as taught by Hetzler, U.S. Pat. No. 5,210,660 where servo information is interspersed with the data stored on each disk surface. The servo sector in sectored servo architecture contains positioning data on each track to help the magnetic head stay on that track. This latter approach is preferred because it can be implemented at low cost without extra components beyond those required for storing data and because it provides the servo information at the data surface which is being accessed, thereby eliminating all thermal sources of track misregistration (TMR).
The use of either sectored servo or dedicated servo surface architectures and the implementation of either of the two are well known to those skilled in the requisite art.
There are also a number of methods used to format disk files, one of which is fixed block architecture (FBA) method which is used in both dedicated servo disk files and sectored servo disk files. In an FBA formatted disk file, each disk track is divided into a number of equal-sized segments, and each segment is divided into sectors containing servo information, identification information (ID), and data.
A typical segment 9 of a track on an FBA formatted disk utilizing sectored servo architecture is illustrated in FIG. 1. The segment 9 comprises sequentially a servo sector 10, and identification (ID) region 11 and a data sector 12. Servo sector 10 further comprises information such as write-read and speed Field 15, address mark (AM) field 16 and position error signal (PES) field 17. The ID region 11, which is written onto the disk during the format operation contains specific information concerning the data sector 12 which can be used during normal operation, either writing or reading, to identify the succeeding data sector 12. The ID region 11 typically comprises a read/write and speed field 18, VCO sync field 19, encoder/decoder flush field 20, sync byte 21, and ID and CRC field 22. The data sector 12 typically comprises fields 23-26 which correspond to the ID fields 18-21, and data and ECC field 27. In a disk file having an ID region, the CHS/ZCHS or LBA information is typically recorded on the data ID field 22 immediately preceding the data sector.
Recently, a new method and system has been developed to increase the capacity of disk drives known as the no-ID format and the disk drive systems utilizing no-ID format are commonly referred to as no-ID disk drive systems. This format has been taught by Hetzler in the aforementioned U.S. Pat. No. 5,500,848, which is assigned to the assignee of the present invention. For no-ID disk drives implementing a sector servo architecture, a "full track number identifier" in the position field in the servo sector of a given track is used in combination with a defect map to uniquely identify the requested data sectors and thereby completely eliminate the use of ID regions.
Briefly stated, bad sectors are mapped out of the disk file by means of a defect map. At disk format time, each sector is written to and read from to determine whether it is usable or defective. Clusters of defective sectors are marked bad by recording in the defect map the sector location identifier of the first bad sector in the cluster and the quantity of consecutive bad sectors in the cluster. During read/write operations, the disk file performs logical block address (customer or system addressable block) to physical block address (total number of sectors available on a disk drive) conversion (logical to physical sector conversion) by searching the defect map for an entry having a value less than or equal to the requested logical sector location identifier. If none is found, the physical block address is equal to the logical block address. If an entry is found, the corresponding offset representing the quantity of consecutive bad sectors is extracted from the defect map and added to the logical block address of the requested sector to produce the physical block address for that sector. The PBA in turn is translated to either CHS (no zone recording) or ZCHS (zone recording present) in order to access a physical disk location.
Once the disk drive completes the required seek operation to the cylinder and head identified, or to the zone, cylinder and head identified, the recording channel scans for the desired data sector by examining the servo sector associated with each data sector as it passes under the head. When the appropriate data sector is found, the data is read and the operation is completed.
In prior no-ID data storage disk drives, having one or two processors (interface processor and servo processors), the PBA of a specific track must be calculated and converted into a CHS or ZCHS value before the actuator is instructed to seek the track. For example, in the case of the two processor design, the interface processor generally performs the logical block address (LBA) to PBA conversion and PBA to CHS/ZCHS for the first possible track where the data is located. The CHS/ZCHS is then used to start a track seek operation by applying necessary signals to servo electronics which in turn provides the necessary current to voice coil which in turn causes the actuator to move.
The process of performing LBA to PBA and PBA to CHS/ZCHS conversions for the first possible track, communicating that information to the servo processor, and then carrying out the seek operation is a time consuming operation and degrades the overall performance of the disk drive. The overall performance of the disk drive is degraded because the aforementioned conversions directly contribute to the track seek time overhead.
Therefore, there is a great need for an invention that can substantially decrease the track seek time overhead in disk drives and further eliminate the overall performance degradation associated with the track seek in no-ID disk drives.